Photoacoustically monitored thermal energy dissipation and xanthophyll cycle carotenoids in higher plant leaves.

When barley leaves were suddenly exposed to strong white light, the rate of thermal de-excitation of the absorbed light energy, measured with a photoacoustic device, increased by around 12% within a few minutes. This phenomenon was paralleled by a quenching of chlorophyll fluorescence. Simultaneous measurements of the heat emission increase and chlorophyll fluorescence quenching allowed the absolute yield phi F of chlorophyll fluorescence to be determined in vivo; phi F varied from around 2% (at Fo) to around 15% (at Fm) in a variety of plant species. No correlation was found between the time course of heat emission increase and the time course of violaxanthin-to-zeaxanthin conversion in barley leaves exposed to various light and temperature treatments, indicating that the zeaxanthin pool built up in the light was not directly involved in the increased heat emission. However, the operation of the violaxanthin cycle accelerated the photoinduced rise in thermal energy dissipation. Photoacoustic measurements on leaves of a zeaxanthin-accumulating Arabidopsis thaliana mutant lacking the violaxanthin cycle indicated that this acceleration could be ascribed to the disappearance of violaxanthin rather than to the formation of zeaxanthin.